Water extraction from floodplain river systems may alter patterns of inundation of adjacent wetlands and lead to loss of aquatic biodiversity. Water reaching the Okavango Delta (Delta), Botswana, may decrease due to excessive water extraction and climate change. However, due to poor understanding of the link between inundation of wetlands and biological responses, it is difficult to assess the impacts of these future water developments on aquatic biota. Large floods from 2009 to 2011 inundated both rarely and frequently flooded wetlands in the Delta, creating an opportunity to examine the ecological significance of flooding of wetlands with widely differing hydrological characteristics. We studied the assemblages of small fishes and microcrustaceans, together with their trophic relationships, in temporary wetlands of the lower Delta. Densities of microcrustaceans in temporary wetlands were generally lower than previously recorded in these habitats. Microcrustacean density varied with wetland types and hydrological phase of inundation. High densities of microcrustaceans were recorded in the 2009 to 2010 flooding season after inundation of rarely flooded sites. Large numbers of small fishes were observed during this study. Community structure of small fishes differed significantly across the studied wetlands, with poeciliids predominant in frequently flooded wetlands and juvenile cichlids most abundant in rarely flooded wetlands (analysis of similarity, P < 0.05). Small fishes of <20 mm fed largely on microcrustaceans and may have led to low microcrustacean densities within the wetlands. This result matched our prediction that rarely flooded wetlands would be more productive; hence, they supported greater populations of microcrustaceans and cichlids, which are aggressive feeders. However, the predominance of microcrustaceans in the guts of small fishes (<20 mm) suggests that predation by fishes may also be an important regulatory mechanism of microcrustacean assemblages during large floods when inundated terrestrial patches of wetlands are highly accessible by fish. We predict that a decline in the amount of water reaching the Delta will negatively affect fish recruitment, particularly the cichlids that heavily exploited the rarely flooded wetlands. Cichlids are an important human food source, and their decline in fish catches will negatively affect livelihoods. Hence, priority in the management of the Delta's ecological functioning should be centred on minimising natural water-flow modifications because any changes may be detrimental to fish-recruitment processes of the system.

Microcrustacean emergence from the dry sediments is an important colonization pathway that allows these microfauna to recover and repopulate temporary aquatic habitats after months or years of dryness. Viable microcrustacean propagules in sediments of three different temporary aquatic habitats – rainpools located within the rarely flooded portions, frequently flooded floodplains and rarely flooded floodplains – were assessed experimentally by flooding the soils. Three major groups of microcrustaceans – cladocerans, copepods and ostracods – emerged from the sediments. Species richness and mean total numbers of emerged microcrustaceans per sample varied across the studied temporary aquatic habitats (Kruskal–Wallis, p < 0.05). Both species richness and mean total number of emerged microcrustacean per sample were lowest in sediments of rarely flooded floodplains. The highest species richness of microcrustaceans emerged from the treatments with soils of the frequently flooded floodplains. The mean total number of emerged microcrustaceans per sample was highest from sediments of rainpools followed by frequently flooded floodplains. Findings of this experiment suggest that high flooding frequency of temporary aquatic habitats may be important to sustain high viability of microcrustacean propagules in the sediments. Reduction of regular inundation of temporary floodplains due to the threats of increasing human water abstractions and climate change may have some negative effects on the microcrustaceans of the Okavango Delta by reducing the viable propagules within these systems. The findings of this experimental study will be relevant towards efforts that are aimed at reconciling anthropogenic pressures and conservation of microcrustaceans in the Okavango Delta. Copyright © 2012 John Wiley & Sons, Ltd.

This paper summarizes the importance of climate on tropical wetlands. Regional hydrology and carbon dynamics in many of these wetlands could shift with dramatic changes in these major carbon storages if the inter-tropical convergence zone (ITCZ) were to change in its annual patterns. The importance of seasonal pulsing hydrology on many tropical wetlands, which can be caused by watershed activities, orographic features, or monsoonal pulses from the ITCZ, is illustrated by both annual and 30-year patterns of hydrology in the Okavango Delta in southern Africa. Current studies on carbon biogeochemistry in Central America are attempting to determine the rates of carbon sequestration in tropical wetlands compared to temperate wetlands and the effects of hydrologic conditions on methane generation in these wetlands. Using the same field and lab techniques, we estimated that a humid tropical wetland in Costa Rica accumulated 255 g C m−2 year−1 in the past 42 years, 80% more than a similar temperate wetland in Ohio that accumulated 142 g C m−2 year−1 over the same period. Methane emissions averaged 1,080 mg-C m−2 day−1 in a seasonally pulsed wetland in western Costa Rica, a rate higher than methane emission rates measured over the same period from humid tropic wetlands in eastern Costa Rica (120–278 mg-C m−2 day−1). Tropical wetlands are often tuned to seasonal pulses of water caused by the seasonal movement of the ITCZ and are the most likely to be have higher fire frequency and changed methane emissions and carbon oxidation if the ITCZ were to change even slightly.

The frequency of fires in the Okavango Delta seasonal floodplains peaked at an intermediate frequency of flooding. Floodplains are commonly burnt every 3–5 years. This study showed fundamental changes in ecosystem properties due to burning. A burnt seasonal floodplain in the aquatic phase had oxygen levels well above saturation, 100–200%, while the levels in the un-burnt control site were below saturation and, at night, could decline to 10–40% saturation. The total phosphorous and total nitrogen concentrations were similar on both floodplains but considerably enriched relative to inflowing water, due to nutrient release from the flooded soil-sediment and animal droppings. Zooplankton biomass was very high in both systems although the abundance of fish fry was ten times higher on the un-burnt floodplain. In a low flood year the un-burnt floodplain water had high nutrient levels, primary production, methane emission, and subsequent uptake of methane in biota, as well as a high zooplankton biomass. The very high flood the following year showed the opposite with much lower production at all levels owing primarily to greater dilution of nutrients. The abundance of fish, however, was much higher during the high flood year. Macrophytes and litter provide direct shelter for fish fry but also promote low oxygen levels when decaying. Large flooded areas result in high fish production by removing obstacles related to congestion. This interplay between hydroperiod and fire may be crucial for the maintenance of high biological productivity both in the aquatic and terrestrial phases in a very nutrient poor wetland landscape. Understanding these interactions is crucial for optimal management.

Communities exposed to intermediate disturbances have been shown to be more diverse than more stable or unstable systems. We recorded the diversity pattern of zooplankton in the Okavango Delta, Botswana, a system which include water bodies with different stability with regard to water levels and wet–dry phases, from permanent rivers and lagoons to seasonal floodplains and temporary water-filled rain ponds. The yearly flood pulse caused a gradual shift in aquatic parameters on seasonal floodplains, which promoted zooplankton diversity. Species composition differed between temporal and permanent habitats, but highest diversity was recorded on floodplains. Diversity on floodplains showed a distinct seasonal trend, being low during increasing flood, to highly diverse during high water periods. Density and hatching sequence of major cladoceran species suggested that the bank of resting eggs in the soil is the major source of species occurrence during flooding. We propose that seasonal floodplains, which have significant higher diversity and abundance, serve as source areas for the cladoceran diversity in the Okavango Delta. From these habitats ephippia are dispersed into the other four habitats. The dominant vectors for such dispersal are probably wind and mammals.

In the Okavango Delta 98–99% of the water from inflow and rainfall is lost to the atmosphere through evapotranspiration. As a consequence 94% of inflowing solutes are retained within the Delta landscape. This process might be expected to result in an entirely saline environment, but that is not the case: the surface waters have very low salinity, supporting a typical freshwater biota. It has been deduced that the numerous islands in the Delta (about 150,000 within an area of 13,500 km2) have been formed through evapotransporative concentration in the groundwater, of infiltrating solutes, followed by precipitation and volume increase. Evidence of this is the large amount of calcrete in island soils. These islands of 3–10 m thickness with clayey soils are underlain by fine Kalahari sand to a depth of 200–300 m, which also indicates that they are formed through surface processes. The infiltration rate of surface water from floodplains and streams into islands is very high, and is predominantly a lateral process that is unidirectional. Evapotranspiration in the riparian woodland zone cause the ground-waters in the central area of islands—with halophyte grasslands—to have very high salinities. By use of chloride as a conservative element the concentration factor between central island groundwater and surface water is calculated to be 500–1,000. This groundwater is depleted of calcium and magnesium supporting the early deductions that these elements have precipitated as calcrete. There is also a large depletion of silicate and potassium that probably have precipitated as well forming the clayey soils typical of the islands. The central island groundwater is dominated by sodium, bicarbonate and dissolved organic matter. The gradual increase of salinity here causes a periodic let off of this water through a density-driven process to deeper layers. This process together with island growth through precipitation of solutes are the two major sink processes of inflowing solutes and explains why the Okavango Delta is at present a freshwater system. The whole island complex is calculated to be 100,000–400,000 years old while some intensely studied islands may be younger: 80,000–240,000 years. The discrepancy is explained by a biassed selection of islands currently in flooded areas with better growth conditions. The uniqueness of the Okavango Delta and ideas for future research are discussed.

. Although wetlands are known to be important sources of dissolved organic matter (DOM) within watersheds, production of DOM within wetlands is not well understood. In the Okavango Delta, a large wetland located in Botswana, large amounts of DOM produced in the wetland are transported in the river network and to the subsurface. The purpose of this study was to gain insight into environmental processing of DOM in wetland surface waters by examining chemical characteristics of plant litter leachates and fulvic acids isolated from two surface water sites in the Panhandle (PHFA) and Seasonal Swamp (SSFA) of the Okavango Delta. Spectroscopic properties measured over the course of leaching experiments indicated a greater abundance of plant-derived DOM over time. Results of elemental and 13 CNMR analyses showed that aromaticities and C:N ratios of PHFA and SSFA and a Cyperus papyrus leachate fulvic acid (CPLFA) were in the range typical for fulvic acids derived from vascular plants. Fluorescence analyses of fulvic acids using parallel factor analysis (PARAFAC) further indicated the importance of plant litter sources in surface water DOM. Environmental processing of DOM in downstream surface waters by bacterial and photodegradation was suggested by higher N and S content for SSFA compared to CPLFA and by differences in δ 15 N, δ 34 S, δ 13 C and fluorescence signatures among the 3 fulvic acid samples. These chemical characterization results suggest that a progressive enrichment of DOM by plant-derived material occurs with distance downstream and that this DOM undergoes some environmental processing within the surface water system.

The species diversity data of seven globally important wetlands (Canadian peatlands, Florida Everglades, Pantanal, Okavango Delta, Sundarban, Tonle Sap, and Kakadu National Park) were compared. The available data for most groups of lower plants and animals are insufficient for a comparative analysis. Data on vertebrates and higher plants are more complete and show high species diversity. The large habitat diversity allows the coexistence of amphibious species with many immigrants from connected deepwater and terrestrial habitats. Several of these immigrant species find an important permanent refuge in the wetlands; some use the wetlands as periodic habitats. All wetlands are important habitats for long-distance migratory bird species. The species composition reflects the biogeography of the respective regions, e.g. the high diversity of large ungulates characteristic for Africa is also found in the Okavango Delta in Botswana, and the high fish species diversity typical for South America is also reflected in the Pantanal in Brazil. The number of endemic species in most wetlands is low, except in the Everglades. The low numbers are explained to some extent by the dramatically changing paleo-climatic conditions that increased extinction rates, but also by the connection with large river systems that act as migratory and transport routes for species from large catchment areas and hinder the genetic isolation of wetland populations. The high number of endemic species in the Everglades is explained in part by its isolation on a peninsula. The relatively low nutrient status of most wetlands does not negatively affect species diversity and often leads to high animal densities. Large populations of endangered or rare species in all wetlands contribute to the great value of these areas for biodiversity protection. All wetlands are subjected to an increasing degree to human pressure through, e.g. water abstraction, changes in the natural flood regime, land reclamation, pollution, over-utilization of natural resources, and poaching. High habitat diversity and a pronounced natural disturbance regime make some of the wetlands vulnerable to invasion by exotic species, as shown for the Everglades. All studied wetlands are at least in part protected by national and international conventions. This provides perspectives for long-term protection only to a limited extent because of major environmental changes in their surroundings. Further strong efforts are required to match protection and sustainable use of the wetlands proper with management activities in their catchments.

In the Okavango Delta (about 28,000 km2) the number of identified species is 1,300 for plants, 71 for fish, 33 for amphibians, 64 for reptiles, 444 for birds, and 122 for mammals. The local occurrence of different species of these taxonomic groups in the Okavango Delta is mainly due to a hydrological gradient from permanent streams and swamps to seasonal floodplains, riparian woodlands, and dry woodlands. This level of species diversity is normal for the southern African region, and all analyzed aquatic groups are composed of ubiquitous species with an additional significant proportion of species originating from northern, more tropical systems. Cyclical variations in climate over thousands of years have created a huge wetland complex in the upper Zambezi and Okavango Rivers during wet phases. This wetland complex has fragmented into the Okavango Delta and other large wetlands in Zambia during dry phases. There are no endemic species in the Okavango Delta while the South-central African wetland complex is a centre of endemism. Species diversity of the Okavango Delta is a consequence of this unique environment, with dynamic shifts in flooding patterns that in turn force constant changes in patterns of plant succession and dependent animals. Temporal variations in flooding also cause accumulation and sudden mobilization of nutrients which are readily used by well adapted plant species. As a consequence, locally high biological productivity occurs, which in turn results in high numbers of grazing mammals.

Water balance in a seasonal floodplain in the Okavango Delta, Botswana was determined for three years (1997–1999). There was no surface outflow, and infiltration to ground water was very large (4.7–9.7 m during 90–175 days of flooding, or on average 4.6–5.4 cm·d−), amounting to 90% of total annual loss of water from the floodplain. At the arrival of the flood, when floodplain ground water was 3–5 m below ground, infiltration was controlled by vertical percolation through the aeration zone and was taking place with rates as high as 1.11–1.74 m during 10 days, or on average 11.1–17.4 cm·d−1. Lateral ground-water flow from the floodplain toward surrounding dryland became the dominant process after the first days of flooding, when the floodplain ground-water table rose to the surface. Lateral ground-water drainage accounted for at least 80% of total infiltration. Direct measurements of infiltration confirmed high rates obtained from the water balance and revealed that the majority of infiltration occurred within a 10-m belt along the shore of the inundated area, with point infiltration rates as high as 42 cm·d−1. The infiltration values are high compared to other large recharge wetlands (e.g., the Everglades, the Hadejia-Nguru) and result from a combination of lack of a low permeability surface layer in the floodplain and strong drainage of floodplain ground water driven by evaporation from the surrounding drylands. High infiltration and lateral ground-water flows have major implications for the Okavango Delta ecology, as they provide water to riparian vegetation, affect floodplain nutrient balance, and are part of the process responsible for immobilization of dissolved minerals.

Groundwater under islands in the Okavango Delta is a known sink of inorganic dissolved minerals, preventing salinisation of this virtually enclosed evaporation-dominated hydrological system. The Okavango Delta is an oligotrophic, yet very productive system, and it is important to understand sources, pathways and recycling of nutrients in order to fully comprehend its ecology. In order to investigate the role of islands as nutrient sinks, concentrations of nitrogen and phosphorus, as well as major inorganic ions were measured in island and floodplain groundwater. The electrical conductivity was found to be up to 50 times higher in the island centre groundwater than in the surrounding floodplain groundwater. The amount of total phosphorus was found to be up to 400 times higher and total nitrogen up to five times higher in the interior of the island than in the surrounding floodplain. These show that major nutrients are, like other inorganic ions, accumulated under islands. Importantly, the ratio of nitrogen to phosphorus was 5:1 in floodplain water and water in island fringe soils, but 1:4 in island centres. This indicates an intensive removal of nitrogen along the floodplain-island groundwater flow path by the floodplain fringe and riparian biota, resulting in a relative enrichment in P. Introduction The Okavango Delta is a large inland wetland fed by the annual flood of the Okavango River. This flood pulse causes the inundated area in the Delta to expand from an annual low of 4,0006,000 km2 to an annual high of 8,000-12,000 km2. The permanent and seasonal flooding creates a wetland ecosystem in stark contrast to the surrounding rain-fed semi-arid savannah of the Kalahari. This makes the Okavango the basis for subsistence livelihoods of the local population, and the main attraction of Botswana’s tourism industry. In 1997 the Okavango Delta was declared a Ramsar site a wetland of international importance. The two primary sources of nutrients in the Okavango Delta are the inflowing Okavango River and atmospheric aerosol deposition. The deposition of atmospheric aerosols is a quantitatively important source of nutrients in the Okavango (Garstang et al, 1998), and one which is characterized by a relatively uniform spatial distribution. N and P in aerosols originate from dust and ash from fires in the region and are primarily transported by large-scale anticyclonic circulation occurring during late winter (August). For the Etosha Pan in Namibia, deposition rates of 0.2-21.7 g·ha–1·day–1 for NO3 and 0.06-1.1 g·ha–1·day–1 for PO4 were calculated based on a large scale atmospheric aerosol mass budget (Swap, 1996). At smaller scales, N and P are redistributed between dry islands and wet floodplains as a result of wind-induced movement of dust (Krah et al, 2004). The concentrations of nitrogen and phosphorus in the Okavango River at the inlet to the Delta are similar to global average concentrations in river water. Cronberg et al (1996) found Botswana Notes & Records, Volume 37 253 ______ 1. All the authors work at the Harry Oppenheimer Okavango Research Centre, University of Botswana, Private Bag 285, Maun,.Telephone: 267 6861833 Fax: 267 6861835. 2. Linkoping University, Sweden. 0.35-0.88 mg·dm–3 of total N with 0.012-0.030 mg·dm–3 of NO3 and 0.024-0.044 mg·dm–3 of total P in the Okavango river water, while the global average is 0.2 mg·dm–3 total N and 0.02 mg·dm–3 of total P (Mitsch and Gosselink, 2000). In the permanent swamp, plant-available N and P compounds brought by the Okavango River are scavenged by bank vegetation, primarily the giant sedge papyrus (Cyperus papyrus) flanking the main distribution channels, when the floodwater seeps from these channels to feed the downstream system of seasonal floodplains (Garstang et al, 1998). Nutrients are very tightly internally cycled by papyrus, being relocated from senescing culms to new growing shoots (Denny, 1985). Small quantities of recalcitrant organic N and P may be trapped in peat. Until the peat is oxidised through exposure to air, or burning, this recalcitrant portion of nutrients is immobilized and is available neither to papyrus nor to downstream seasonal floodplains, aside from a fraction which may be microbially mineralised. The N and Ppresent in the water of the seasonal floodplains downstream, therefore, have either escaped nutrient “stripping” by aquatic macrophytes taking place in the permanent swamp by being in non-plant available forms, were released from senescing plant material along the flow path, or were delivered to the floodplains as atmospheric deposition. The contributions of these various sources, however, have not to date been quantified. The water of the seasonal floodplains is characterised by a relative enrichment of both N and P compared to the main Okavango River. In the floodplains of the middle Boro system values of 0.98-3.1 Botswana Notes & Records, Volume 37 254 Figure 1. The Okavango Delta, Botswana. mg·dm–3 of total N (0.016-0.183 mg·dm–3 of NO3) and 0.042-0.466 of total P were recorded by Cronberg et al (1996) and Hoberg et al (2002). In the seasonal floodplains there is an intensive cycling of nutrients controlled by the seasonal cycle from reducing to oxidising conditions in the sediments. During the flood recession, under oxidising conditions, the organic (soluble and to a lesser extent particulate) N and P (ON and OP) present in the senescent plant material and in the soil are mineralised by microbial action and oxidation into the plant-available forms PO4 and NO3. The ON is transformed in a two-stage process via NH4 (ammonification) and thence to NO2 and NO3. OPis transformed by degradation of organic material and then by oxidation and some microbial action to form soluble PO4. At the arrival of the flood, these enter into solution in addition to the P and N brought in with the flood water. Phosphorus cycling is not significantly microbially mediated in anaerobic conditions. When oxidising or low pH conditions pertain, P is in the form PO4, orthophosphate, or the form soluble organic phosphate (SOP). Orthophosphate tends to form strong complexes with clays, calcium/magnesium carbonates, and metal hydroxides (Fe or Al); these complexes effectively immobilise P, making it unavailable to plants, but also removing it from solution (Axt and Walbridge, 1999; Mitsch and Gosselink, 2000; Wetzel, 2001). SOP, having no charge, is not prone to complexing, and although soluble is not a plant-available form. During the flood, N and P are actively taken up in the floodplain sediments through processes of complexation, microbial activity, and directly from pore water by rooted emergent plants. Additionally, infiltrating flood water carries dissolved N and P to the island fringe soil supplying bacterial processes in the hyporheic zone. Some N may be mineralized into to gaseous NH3, N2O and N2 and effectively removed from the system. All these processes contribute to a decrease in total N concentration in floodplain water, a phenomenon observed by Hoberg et al (2002) and Krah et al (2005). One of the paths of movement of nutrients from the seasonal floodplains in the Okavango Delta mentioned above is advection with infiltrating water. Infiltration is an important element of floodplain water balance: up to 90% of floodplain water input can be removed that way (Ramberg et al, 2005). Some nutrients carried by the infiltrating water may be utilized by bacteria in the hyporheic zone or in shallow groundwater in the direct vicinity of floodplains. For example, Ramberg et al (2000) showed that there were high concentrations of Nitrosomonas and Nitrobacter, which produce plant available NO3 from organic N, in the groundwater along the perimeter of an island (Figure 2). In the Okavango Delta, advection and movement with groundwater is the process responsible for immobilization of inorganic chemicals described in detail by McCarthy and Ellery (1994). In this process, a cone of groundwater depression under an island is created by island vegetation transpiring shallow groundwater (Figure 3). This induces radial flow of groundwater towards the island centre, maintained by infiltration of surface water in the surrounding floodplains. Concentrations of solutes in the island groundwater are subject to evaporative and transpirative enrichment, and as a result, high salinity brines are formed in the island centre. A groundwater salinity gradient develops, which affects island vegetation: the island centre is bare or supports only salinity tolerant grasses, while the island fringe, where groundwater is fresh, supports lush riparian woodland. Concentrations of solutes often exceed their saturation points, and precipitation of carbonates and silicates takes place, contributing to island aggradation. The process is ubiquitous in the Okavango Delta as is manifest by the conspicuous zoning of vegetation on virtually all islands in the system. The process is thought Botswana Notes & Records, Volume 37

In the Okavango Delta in Botswana, dissolved organic matter (DOM) transport is controlled by the slow movement of an annual flood ‘pulse’ across permanently and seasonally flooded wetlands, known respectively as the Permanent Swamp and Seasonal Swamp. We studied temporal and spatial variations in fluorescence index (FI) and specific UV absorbance (SUVA) of DOM to identify DOM sources and fate during the flood. Dissolved organic carbon (DOC) concentrations ranged from 2 to 25 mg C L−1 in channels of the Delta, with seasonal floodplains having consistently higher concentrations. Chemical indices, such as DOC concentrations, conductivity, specific UV absorbance (SUVA), fluorescence, total dissolved nitrogen, and chlorophyll a, were analyzed for channel and floodplain sites in the Seasonal Swamp. DOC concentrations increased during the rising limb of the flood in the Seasonal Swamp. SUVA of whole water samples and fluorescence index (FI) of fulvic acids isolated from channel and floodplain sites changed in a manner indicating the release of DOM by leaching of plant litter during the flood. After the flood receded, DOC concentrations and fulvic acid content decreased, and microbially-derived sources of organic matter dominated. Along two river reaches, measuring over 400 km each, variations in DOC concentrations were primarily due to geomorphology, with the effects of the annual flood overprinted atop the spatial controls. Increasing downstream DOC concentrations were found to be a product of inundation of DOC-rich seasonal floodplains and evaporation-enriched waters downstream. Increasing SUVA, dissolved nitrogen, and fulvic acid content, and decreasing FI downstream suggested microbial processing of terrestrial DOM and possible release of nutrients incorporated in the DOM.

A fast and simple method to quantify filamentous algae in inverted microscope is described. It is based on the number of intersections between the midline of a diametrical transect and the filaments on the chamber bottom. It is shown to be theoretically sound and is compared with the common length measurement method.